1. Field of the Invention
The present invention relates to the measurement of strains on and within structures using fiber optic sensors.
2. Background of the Invention
Fiber optic sensors have been used for many years to measure parameters such as strain, temperature, stress, displacement, acceleration, and other physical and chemical properties of structural components. The inherent advantage of fiber optic sensors is their immunity to electromagnetic interference (EMI) and their resistance to corrosion. Additional advantages include their small size, large band-width, and low signal loss.
Optical fiber sensors can be categorized according to the approach used to modulate and demodulate the optical signal. Five common modulation and demodulation techniques are time delay modulation, amplitude modulation, phase modulation, polarization modulation and wavelength modulation. Each technique has its advantages and disadvantages for particular applications. Time-delay modulation is most advantageous for monitoring strain in large structures over periods of time as long as 10 to 40 years.
U.S. Pat. No. 4,928,004 to Zimmermann, et al. ("Zimmerman I"), which is incorporated herein by reference, discloses monitoring strain in a structure using an optical fiber, with discrete reflective markers along its length, embedded in the structure or attached to the structure. The time delay between adjacent reflections from the reflective markers is a measure of the absolute fiber length of any given segment. Thus strain in the structure can be monitored by monitoring changes in time delays.
This approach is very limited due to the resolution with which time delay can be currently measured. The time delay between measurements can be measured using an Optical Time Domain Reflectometer ("OTDR"). However, the most advanced OTDR cannot measure time delay with a resolution much better than .+-.1 picosecond (ps). This corresponds to a strain measurement resolution of approximately .+-.130 130 microstrain (.mu..epsilon.). Many applications require a strain measurement resolution of .+-.10 .mu..epsilon., i.e., at least one order of magnitude better than .+-.130.mu..epsilon..
U.S. Pat. No. 5,189,299 to Zimmermann, et al. ("Zimmerman II"), discloses increasing the interaction length of the optical fiber by using fiber optic "re-entrant loops." These loops direct the optical signal through the same length of optical fiber many times, thereby causing the optical signal to experience the effects of the parameters being measured (e.g., strain) many times as well. Zimmerman II uses special "tap-off" couplers to split off and re-circulate portions of the optical signal. The end result is improved measurement resolution because the optical signal propagates through the sensing region many times, not just twice (as in Zimmerman I). The most important drawback of the re-entrant loop technique is that multiple sensors cannot be connected in-line to monitor different sections of a structure. Furthermore, the tap-off couplers have excessive loss characteristics, severely attenuating the optical signal.
U.S. Pat. No. 5,201,015 to Von Bieren, et al. ("Von Bieren"), discloses the use of pre-tensioned fiber optic "coils" to form an interferometric sensor. Von Bieren's sensor, however, requires the use of relatively expensive special elliptical core fibers, and requires imposing a bias strain on the fiber coil. Furthermore, Von Bieren does not allow serial connection of multiple coils.